1. Field of the Invention
The present invention relates to the field of building construction, and more particularly to a novel steel moment resisting frame (SMRF) connection. Such SMRF connections are used in the construction of both single and multi-story structures of either original or retrofit construction.
2. Brief Description of the Prior Art
The prior art contains many teachings for the construction of moment connections and other related structural steel joints. These teachings have either focused on connections that allegedly reduce construction costs and facilitate erection methods, or on improving the seismic energy absorption capability of isolated load transfer mechanisms in a given joint while ignoring other critical load transfer mechanisms that are required to complete the SMRF system. Quite importantly, given the severe lessons learned in recent major earthquake activity, prior art SMRF connections are not suitable for use in seismically active areas. Examples reside in U.S. Pat. Nos. 3,952,472, 4,094,111 and 4,993,095. U.S. Pat. No. 3,952,472 to Boehmig teaches a relocatable joint connecting a horizontal beam and a vertical column. Boehmig teaches a moment connection (weld connection) between the end of his horizontal beam and the end of his parallel plates. His parallel plates are attached to the column. Boehmig's beam does not extend between the parallel plates, i.e., Boehmig's parallel plates do not overlap, or extend along the sides of, the horizontal beam as they do in the invention herein.
Prior to recently reported earthquake damage, the traditional beam-to-column SMRF connection used in most steel frame buildings was comprised of a full-penetration single-bevel groove weld connecting both beam flanges of a horizontal beam to the vertical column flange to resist earthquake lateral forces in rigid joint/moment frame action. The gravity forces were resisted by a shear-resisting tab plate that was shop welded to the column flange and field bolted in single shear to the beam web using high strength bolts.
The design approach adopted by the structural engineering community for SMRF systems in seismic areas assumes that a significant level of system ductility can be developed. This ductility is available in steel ductile frames if premature brittle failures are prevented. Testing to date of SMRF connections, following recent severe earthquake activity, suggests that the behavior of beam-to-column joints will depend on the strain rates imposed on the more brittle load transfer mechanisms along the load path.
Observed recent earthquake damage to SMRF connections consists primarily of either a partial or complete failure of the full penetration single-bevel groove weld between the beam flange and the column flange, either in the weld itself or along the heat affected zone of the column flange, pulling with it a divot of parent column steel from the face of column flange. The origination of the crack is normally at the narrow root of the groove weld profile, which is inherently subject to slag inclusions during the field welding process. These inclusions act as stress risers that initiate cracking during the impactive load from an earthquake. Stress risers are also created by the backer bar used to bridge the root gap before making the weld. The backer bar is commonly tack welded in place below each beam flange and not removed. In addition, these beam flange-to-column flange failures have resulted in shear failure of the high strength bolts connecting the beam web to the shear tab plate attached to the column flange for the support of gravity loads.
in other instances, the crack again originates at the root of the groove weld, but enters the column flange and propagates through the full thickness and width of the flange and into the column web. This particular cracking pattern appears to be more pronounced in the jumbo column sections, both rolled and built-up sections.
The effect of these recent SMRF connection failures in damaged buildings is three-fold; 1) the integrity of the seismic lateral load resistance of the connections has been seriously compromised, potentially leading to the loss of gravity support and partial collapse of the building during extended strong ground motion or aftershocks; 2) building owners and commercial property insurance carriers have lost confidence in the earthquake performance of steel buildings, and 3) the International Conference of Building Officials has issued an emergency code change that deletes the prequalified SMRF connection because of poor performance of steel moment frame beam-to-column connections in recent earthquakes and subsequent testing at the University of Texas, at Austin.
In response to this building industry crisis, practicing structural engineers, together with university researchers, metallurgical and welding engineers, steel and welding electrode manufacturers, and steel fabricators and erectors individually and collectively appear to be largely focused on ways to modify the traditional SMRF connection configuration. These modifications to the traditional SMRF connection unfortunately still rely fundamentally on the post-yield straining of large highly-restrained full-penetration single-bevel groove welds (performed under hard-to-control field conditions which can dramatically affect weld toughness) or structural steel column shapes in a through-thickness direction (i.e., 90.degree. to the longitudinal direction of the weld or normal to the rolled grain of the steel shape), under the influence of impactive earthquake forces. As clearly demonstrated by the recently observed and reported widespread damage and subsequent testing, these joint configuration attributes do not provide a reliable mechanism for the dissipation of earthquake energy, and can lead to brittle fracture of the weld and the column. Brittle fracture is in violation of the SMRF design philosophy as codified in the Uniform Building Code. Hence the need for a novel SMRF beam-to-column connection that altogether eliminates these negative attributes, which is the subject of this invention.